DURHAM, N.H. -- Changes to farming practices in rice paddies in China
may have led to a decrease in methane emissions, and a decline in the
rate that methane has entered the Earth's atmosphere over the last 20
years, a NASA-funded study finds.

Lead author Changsheng Li, professor of natural resources at the University
of New Hampshire's Institute for the Study of Earth, Oceans, and Space,
notes that in the early 1980s Chinese farmers began draining their paddies
midway through the rice growing season -- replacing a strategy of continuous
flooding -- when they learned it would increase yields and save water.
Draining soil stimulates rice root development and accelerates decomposition
of organic matter in the soil to produce more inorganic nitrogen, an important
fertilizer.

As an unintended consequence, less methane was emitted out of the rice
paddies. Methane is produced by soil microbes in paddy soils under anaerobic
conditions, or in the absence of air or free oxygen. Midseason drainage
aerates the soil again, and hence interrupts methane production.

Over 100 years, methane is 21 times more potent as a greenhouse gas than
carbon dioxide (CO2). Since 1750, methane concentrations in the atmosphere
have more than doubled, though the rate of increase has slowed during
the 1980-90s. Currently, about 8 percent of global methane emissions come
from the world's rice paddies.

"There are three major greenhouse gases emitted from agricultural lands
-- carbon dioxide, methane and nitrous oxide," said Li. "Methane has a
much greater warming potential than CO2, but at the same time, is very
sensitive to management practices."

Li and his colleagues recorded reductions in methane caused by draining
practices at several experimental sites in China and the U.S. At the same
time, they observed that the amounts of methane reduction varied greatly
in space and time due to complex interactions among several factors.

The researchers spent more than 10 years developing a biogeochemical
model, called the Denitrification-Decomposition (DNDC) model, which would
handle the major factors relating to methane emissions from rice paddies.
These factors included weather, soil properties, crop types and rotations,
tillage, fertilizer and manure use, and water management. The model was
employed in the study to scale up the observed impacts of water management
from the local sites to larger regional scales.

Remotely sensed data from the NASA/U.S. Geological Survey Landsat Thematic
Mapper (TM) satellite were utilized to locate the geographic distributions
and quantify the acreage of all the rice fields in China. A Geographic
Information System database amended with this Landsat data was constructed
to support the model runs at the national scale and to predict methane
emissions from all rice fields in the country.

The researchers adopted 1990 as a mean representative year as they had
detailed, reliable data for that year, and then ran the model with two
water management scenarios to cover the changes in farming practices from
1980 to 2000. The two scenarios included continuous flooding over each
season, and draining of paddy water three times over the course of each
season.

When the two model runs were compared, the researchers found that methane
emissions from China's paddy fields were reduced over that time period
by about 40 percent, or by 5 million metric tons per year -- an amount
roughly equivalent to the decrease in the rate of growth of total global
methane emissions.

"The modeled decline in methane emissions in China is consistent with
the slowing of the growth rate of atmospheric methane during the same
period," Li said. "Still, more work will be needed to further verify the
relationship demonstrated in this study with limited data points."

Demand for rice in Asia is projected to increase by 70 percent over the
next 30 years, and agriculture currently accounts for about 86 percent
of total water consumption in Asia, according to a recent report from
the International Rice Research Institute. Changes to management practices
like this will be more important and likely in the future as the world's
water resources become increasingly limited, Li said.

"Just like the Chinese farmers did, if farmers around the world change
management practices, we can increase yields, save water and reduce methane
as a greenhouse gas," Li said. "That's a win-win situation."

The study, which appears in the print version of Geophysical Research
Letters in late December, was funded by NASA through grants from the multi-agency
Terrestrial Ecosystems and Global Change Program, and also NASA's Earth
Science Enterprise.